High temperature combustion of fossil fuels leads to the formation of a variety of oxides of nitrogen such as nitric oxide (NO) and nitrogen dioxide (NO.sub.2) collectively known as NO.sub.x. The oxidation of nitrogen (in the combustion air) and the fuel-bound nitrogen leads to the formation of NO.sub.x. High combustion temperatures lead to the formation of NO (observed in pulverized coal combustion). The oxidation of the emitted NO by atmospheric oxygen and ozone (photocatalytic reaction) leads to the formation of nitrogen dioxide (NO.sub.2) at ambient temperature in the atmosphere. A third type of nitrogen oxide (N.sub.2 O) is formed by low temperature coal combustion (such as in fluidized bed combustors).
These gases have adverse effects on human and plant life and create well-documented pollution problems. NO forms methemoglobin in blood thereby reducing its oxygen carrying capacity. NO.sub.2 is the leading cause of smog and ozone that attack the respiratory tract. It also leads to the formation of acid rain. N.sub.2 O has a long life in the atmosphere and its accumulation increases the heat retention capacity of the atmosphere through the greenhouse effect. Given these adverse effects, state and federal regulations to curb the emission of NO.sub.x have been enacted. The first of these came into effect in 1969 in Ventura County, California and have continuously become more stringent.
The emission of NO.sub.x has exceeded 20 million tons annually in the US alone. Statistics show that 45% of this amount is emitted by trucks, and 32% of is emitted from thermal power plants. Internal combustion engines, industrial boilers, process heaters and gas turbines make-up the balance [Baumbach, G., Air Quality Control, 1996]. The regulatory bodies have targeted the major sources of NO.sub.x such as stationary power plants and automobiles for the past 15 to 20 years. Regulations are being enacted to continually bring down the level of emitted NOx. Recently, 392 power plants in 22 states were ordered to curtail the NO.sub.x emissions by 50% by March 2003. This translates to a reduction in the NO.sub.x emitted by about 500,000 tons at a cost of about $2,000/ton NO.sub.x reduced. In the face of the impending, stringent NO.sub.x regulations, economical NO.sub.x removal from flue gas is thus essential for the long-term economic viability of the fossil fuel based thermal power plants.
Extensive investigations have been done in the area of NO.sub.x abatement. Primary abatement measures target the reduction of NO.sub.x in the combustion unit. These techniques involve lowering the combustion temperature by staged combustion, burner out of service (BOOS), lower air preheating, flue gas re-circulation and the use of low-NO.sub.x burners [Muzio, L. J. and Quartucy, G. C., Prog. Energy Combustion Science, 23, 233-266, 1997]. Although these modifications are relatively inexpensive, the percent NO.sub.x reduction achieved by these primary measures is only 35-45%, and they are thus unable to achieve compliance. Additional fuel can be injected over the combustion zone to create a reducing atmosphere where the fuel (coal/gas) reacts with NO to form N.sub.2 and CO/CO.sub.2 at high temperature of 1100.degree. C. [Chen, W. and Ma, L., AlChE J., 42(7), 1968-1976, 1996; Burch et al., Combustion and Flame, 98, 391-401, 1994]. This re-burning technique suffers from the potential formation of side products such as HCN and NH.sub.3.
To achieve compliance, secondary measures such as Selective Non-Catalytic Reduction (SNCR) were introduced in 1970. SNCR involved the reduction of NO to nitrogen gas using reducing agents such as ammonia and urea at an optimum temperature in the 850-1000.degree. C. range. This technique, being temperature sensitive, leads to ammonia slippage (at lower temperature) and conversion to NO itself (at higher temperature) [Gullet et al., Ind. Eng. Chem. Res., 31(11), 1992]. Complimentary processes such as Selective Catalytic Reduction (SCR) achieve similar reduction by catalysis. Some of the common catalysts employed being molecular sieves, metal and metal oxides, supported on alumina, silica or titania. These catalysts reduce the operating temperature of the reduction processes from 850-1000.degree. C. down to 280-450.degree. C. The SCR technique entails huge capital and operational costs due to the additional reductant and catalyst requirements [Cho, S. M., Chem. Eng. Progress, January, 1994]. Other relatively benign reductants like CO, H.sub.2, CH.sub.4 and acetone suffer from higher selectivity to oxygen in the flue gas [Tsujimura et al., J. Chem. Eng. of Japan, Vol. 16, No. 2, 1983; Jang et al., Energy and Fuels, 11, 299-306, 1997]. Mature SCR technologies also suffer from gas phase poisons such as sulfur dioxide and arsenic, which lead to the formation of ammonium bisulfite and oxidize sulfur dioxide to SO.sub.3, complicating SO.sub.x removal downstream. Being temperature sensitive, these technologies do not adapt well to changing boiler load conditions.
Carbon-based technologies have also been used for NO.sub.x reduction. At high temperatures, micronized coal has been demonstrated as a re-burning fuel in fossil fuel fired boilers to reduce NO. Combined SO.sub.x /NO.sub.x processes have been developed where carbon is used as a catalyst for the reduction with ammonia at temperatures below 200.degree. C. [Hjalmarsson, A. K., NO.sub.x Control Technologies for Coal Combustion, IEACR/24, IEA Coal Research, 1990]. However, recent studies have spurred the development of another carbon-based technology.
Below ambient temperature, NO adsorbs physically and reversibly on carbon, and the adsorbed NO can be released via thermal desorption. However, the concentration of physically adsorbed NO falls with rising temperature and this phenomenon is virtually nonexistent in the range of temperature of greatest interest (300-800.degree. C.) [Teng, H. and Suuberg, E. M, Ind. Eng. Chem. Res., 32, 416423, 1993]. In the temperature range of greatest interest, Smith et al. in J. Phys. Chem., 63, 544 (1959), carried out one of the seminal works. They showed that below 200.degree. C., NO forms nitrogen molecules and carbon-oxygen complexes by the dissociative chemisorption of NO. The activation energy for the chemisorption was determined to be 15-18 kcal/mole. Further NO reduction does not occur at the low temperature due to the saturation of the surface. A continuous sustained reaction proceeds only at a higher temperature (450.degree. C. and above) because of the creation of fresh active sites by continuous thermal desorption of these complexes. The products of the reaction are N.sub.2, CO and CO.sub.2. The regeneration of carbon requires higher activation energy of 45-60 kcal/mole. It has been observed that the complexes desorbing from the surface are in the form of CO and CO.sub.2. Based on these observations, it has been widely proposed that the complexes formed as a result of chemisorption be denoted as C(O) and C(O2). It has also been observed that the CO.sub.2 starts evolving at a lower temperature than the CO species [Furusawa et al., International Chemical Engineer, Vol. 20, No. 2, 1980; Chan et al., Combustion and Flame, 52, 37-45, 1983]. Once the spent carbon has been regenerated, the NO.sub.x reduction can proceed again. This hypothesis is accepted widely even today [Illan-Gomez et al., Energy and Fuels, 10, 158-168,1996].
Some inorganic species have been known to catalyze the NO/carbon by lowering the reaction temperature. These inorganic constituents could be either inherently present or deliberately added to the carbon matrix. Chan et al. (see above cited reference) observed that the char with high ash content catalyzed the NO.sub.x reduction. They noted that the ash, rich in ion exchangeable calcium might be responsible for the catalytic effect. Alkali and alkaline earth metals have been proven catalysts in coal gasification, water gas shift reaction and methanation of CO which necessitate oxygen transfer between the gaseous reactant and carbon. With this premise, studies have been done on carbon impregnated with K, Ca, etc. The catalytic role of calcium in char oxidation has been well established through a series of studies involving impregnation/ion-exchange techniques [Radovic et al., J. Catal., 82, 382, 1983; Hengel, T. D., and Walker P. L., Fuel, 63, 1214, 1984; Levendis et al., Ener. Fuels, 3, 28, 1989; Gopalakrishnan et al., Energy and Fuels, 8, 984, 1994]. Researchers have shown that by integrating these inorganic species into carbon matrix the NO-carbon interaction takes place via an alternate pathway thus reducing the temperature of chemisorption [Kapteijn et al., J. Chem. Soc., Chem. Commun., 1084, 1984].
It is known that the O.sub.2 /char interaction rate is much higher than the NO/char reaction [Chan et al. in above cited reference]. The presence of oxygen in the range of 0.1-2% enhances NO reduction. This occurs by the low temperature gasification of carbon by oxygen leading to the creation of active sites [Suzuki et al., Ind. Eng. Chem. Res., 33, 2840-2845, 1994]. But in the presence of 5% oxygen, the carbon starts reacting with the oxygen preferentially leading to a loss in carbon and thus a loss in the overall reduction of NO. The influence of metal impregnants on NO-carbon reaction in the presence of oxygen has also been studied. Copper (impregnated as Cu), Ni [Yamashita et al., Applied Catalysis, 78, 1991], calcium (in the form of CaO) [Yamashita et al. in above cited reference; Illan-Gomez et al. in above cited reference] and potassium [Illan-Gomez et al. in above cited reference] have all been shown to promote reactivity of carbon. Yamashita et al. (in above cited reference) have shown that with the metal-catalyzed carbon, the C--NO reaction takes place at much lower temperatures of about 300-500.degree. C. and is promoted by the presence Of O.sub.2. The reduction of NO was further enhanced in the presence of a metal, with the order of reactivity being Ni&gt;Ca&gt;Cu in the absence of oxygen and Cu&gt;Ca&gt;Ni in the presence of oxygen. As in the case of un-catalysed reaction, it was seen that the presence of oxygen in minute quantities enhanced the NO reduction while higher oxygen concentrations led to a loss in carbon due to the increased reaction of carbon with oxygen. The presence of surface species such as Cu.sub.2 O and CuO proves that the pathway for the oxygen molecules reaching the active sites on carbon has been altered.
Given the vast literature available, the focus of the present invention was to enhance the selectivity of the carbon-NO reaction in the presence of oxygen. The present invention aims to quantify the effect of the various gaseous (gas concentrations, reaction temperature) and solid (type of carbon, surface area, impregnate type, extent of impregnation, etc.) operating parameters on the selectivity. The experiments did not involve any pretreatment of the carbon involved so as to mimic actual operating conditions in a power plant and minimize pretreatment cost.
A variety of carbonaceous materials have shown NO reduction potential in the presence of oxygen. These include high cost commercially available activated carbons and low cost coal chars. Based on the experimental results obtained, it can be concluded that impregnation of carbon is beneficial in catalyzing the various gasification reactions and lowering the temperature of operation. Calo et al., Energy and Fuels, 13, 761-762, 1999, have shown an increase in the rate of NO reduction with increasing surface area as expected. Results obtained in the present invention show that besides the increase in the rate of the NO reduction, the selectivity of the carbon-NO reaction is enhanced as well by the use of a high surface area carbon. Similar conclusion can be drawn about the effect of temperature. Although an increase in temperature enhances both the carbon-NO and the carbon-oxygen reaction, the present invention shows a preferential increase in selectivity of the carbon-NO reaction with increasing temperature. Despite the parasitic consumption of char by oxygen, the requirement of char is only about 8-15 g carbon/g NO reduced. The reasonable requirement of char and the low cost and wide availability of high alkali lignite coal to make these activated chars provides the economic incentive to further develop this technology.
Accordingly, it is an object of the present invention to provide a method for NO.sub.x removal capable of achieving compliance with current environmental standards, with many of the cost and inefficiency disadvantages of prior art methods.
The above cited references are hereby incorporated by reference.